Four wheel steering systems in motor vehicles typically involve a steering control system for the vehicle rear wheels in addition to the conventional steering control of the vehicle front wheels. Rear wheel steering control is typically used, for example, to reduce vehicle yaw (i.e., deviation from the intended course) in lane change maneuvers. Rear wheel steering control can also increase the stability of a loaded vehicle, such as a vehicle towing a trailer, by reducing trailer sway. The widespread use of higher center of gravity vehicles in recent years has led to the further development of steering systems that control the handling of this type of vehicle.
Variables such as vehicle speed and axle loading can directly affect the type of steering control that is generally desired for safe and efficient handling of a vehicle. Therefore, four wheel steering (4 ws) systems are typically calibrated for different load and speed conditions. That is, one steering control calibration can be set for a normal load category, such as curb weight, and another steering control calibration can be set for a tow load category, such as a trailer towing condition.
However, there are typically significant loading variations within each calibration category. For example, the normal load category may have a weight variation of approximately 2,000 pounds, while the tow load category may include a trailer weight range of approximately 500 to 10,000 pounds. In order to accommodate load variations such as these within a calibration category, 4 ws systems may have their steering calibrations adjusted in order to obtain a performance compromise that can adequately cover the load extremes of that calibration category. For example, the steering calibration for the normal load category may be tuned for reasonably good high speed handling capability at the high weight end of the range, and for acceptable steering sensitivity at the low weight end of the range. Similarly, tuning compromises may also be made in the tow loading category, in order to provide a reasonably satisfactory steering calibration for the wide range of trailer weights, or other towed loads, such as boats and the like.
For a driver, however, the selection of an appropriate steering calibration category (normal load or tow load) may not be obvious in certain types of situations. For example, if the driver wants to tow a light trailer, he may find that the normal load calibration category actually provides better steering performance under certain driving conditions than the tow load calibration category. On the other hand, the selection of normal load steering for a towing situation may not provide some of the vehicle handling capabilities that would otherwise be available with the selection of the tow load category.
Accordingly, it is desirable to provide an improved steering system that incorporates the actual loading condition of the vehicle in the steering calibration process, in order to achieve a more optimal degree of steering control. In addition, it is desirable to provide the improved steering system capabilities with minimal impact on the system hardware requirements. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.